YouTube had a UI overhaul in Sep 2017, and on top of Electrolysis Firefox coming Nov 2017, it's a really hectic time to find any working add-ons these days.

It's very simple to use, just follow those 3 screenshots.

It works on both Sep 2017 UI and "old YouTube" UI.

The only reason i did not give 5-star is the links seem to be generated / provided by a third-party website, and not by this add-on author. So, it keeps me worried from time to time which one will go out of service / update / etc.